Texas Instruments Incorporated of Dallas, Tex. has developed semiconductor chips containing an array of individually controllable mirrors (substantially square and/or diamond shaped) having a reflective surface. More specifically, the DMD is revolutionary in that it is truly a digital display device in an integrated circuit solution. The DMD is an electro/mechanical/optical SLM (Spacial Light Modulator) now being used to provide images for projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit SLM comprised of a high density array of square or diamond shaped moveable micromirrors. The micromirrors have an X and Y surface dimension of between about 9 micrometers and 17 micrometers. These mirrors are fabricated over address circuitry including an array of RAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bi-stable. That is, stable in one of two positions wherein a source of light directed upon the mirror array will be reflected in one of two directions. In the stable or “on” mirror position, incident light to the mirror will be reflected to a projector lens and focused on a display screen or a photo-sensitive element of a printer. In the other or “off” mirror position, light directed on the mirror will be deflected to a light absorber. Further, each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. The projector lens ultimately focuses and magnifies the modulated light from the pixel mirrors onto photoresistive medium or a display screen and produces an image. If each pixel mirror of the DMD array is in the “on” position, the displayed image will be an array of bright pixels. As is discussed below, each of the mirrors is supported by a pair of torsional hinges. However, it should be understood that although the invention is discussed with respect to mirrors, the invention could also be used with other microdevices requiring an array of structures supported by torsional hinges.
As detailed and commonly assigned in U.S. Pat. No. 5,535,047 entitled “Active Yoke Hidden Hinge Digital Micromirror Device” to Hornback, and shown in FIG. 1 of the present application, there is disclosed a digital micromirror device (DMD) spatial light modulator shown at 10. DMD 10 is a single chip integrated circuit that includes an array of micromirrors 20 monolithically fabricated over a memory cell array formed upon a substrate. As shown, each pixel mirror 20 includes a square or diamond shaped mirror supported upon and elevated above a butterfly shaped yoke generally shown at 22 by a rectangular support post 24. Support post 24 extends downward from the center of the mirror 20 and is attached to the center area 26 of the yoke 22 along the torsional axes 28 (indicated by dashed lines), as shown, to balance the center of mass of mirror 20 on yoke 22. Yoke 22 is actually supported at the center area 26 along torsional axes 28 by a pair of torsional hinges 30. The other end of each torsional hinge 30 is attached to and supported by a hinge support post cap 32 defined on top of a respective hinge support post 34. A pair of elevated mirror address electrodes 36 and 38 are supported by a respective address support post 40. The address support posts 40 support the address electrodes 36 and 38 away from and above a biased/reset bus, at a pair of substrate level address electrode pads 42. In a similar manner, the hinge support posts 34 support the torsional hinges 34, which in turn support the yoke 22. To selectively reflect light, mirror 20 and yoke 22 are together rotated about the torsional axes 28 of the yoke 22, defined by the hinges 30.
As the size of the individual pixels have shrunk below 16 micrometers, the electrode support post 40 and the support post 34 used to raise the reflective structure above electrode pads 36 and 38 on the substrate level, have developed integrity issues. More specifically, the hinge support post 34, which support the torsional hinges 30 and the electrode support post 40, are subject to corrosive structure damage during the fabrication processes.
Referring now to FIG. 2, there is a cross-section illustration showing the structure damage suffered during such fabrication. As shown, substrate 50 includes an electrode layer 52 which will be patterned and etched to define individual electrode pads, such as for example electrode pads 44 and 46 discussed above with respect to FIG. 1. A sacrificial spacer layer 54 is deposited over the electrode layer 52. The spacer layer 54 is then patterned and etched through to the electrical layer 52 to form hinge post apertures, such as hinge post aperture 56. A metal layer 58 is deposited over the top surface 60 of the spacer layer 54 in the sidewalls of the aperture 56. The metal layer 58 when originally deposited also covered the top or exposed surface of the electrode layer 52 (not shown), which is at the bottom of the aperture 56. However, the metal layer on the bottom of the aperture is subsequently removed during an etching process. The metal layer 58 on the sidewalls 62 of the hinge post aperture is also covered with an oxide layer 64, which provides additional support. The oxide layer 64 is originally deposited over the bottom 66 of aperture 56 and the top surfaces of the metal layer 58 as well as the sidewalls of aperture 56. However, as discussed below, the oxide is removed everywhere except the sidewalls of the aperture by etching.
More specifically, the process of patterning and etching the metal layer 58 includes the deposit of between about 2,000 Å and 5,000 Å layer of oxide over the metal layer including the metal covering the hinge post aperture 56. The oxide layer 64 is anisotropically etched, which, as will be appreciated by those skilled in the art, leaves the oxide layer 64 on vertical surfaces such as the sidewalls of the hinge post aperture. The oxide layer on the sidewalls array provides additional support to the hinge post.
Although not a significant problem with the 16 micrometer mirrors, the size reduction of the newer and smaller mirrors means that because of the very small dimensions of the aperture entrance at the top surface of the aperture, only a very thin layer of the protective oxide layer is deposited on the bottom 66 of the hinge post aperture and may be completely removed during the anisotropic etch. Likewise, as can be seen in the illustration of FIG. 2, the oxide layer thickness on the sidewalls is significantly decreased at the bottom of the aperture. In addition, the very thin layer of oxide often includes pores or holes that may extend completely through the thin layer. Consequently, during the patterning of the metal layer, the extremely corrosive developing solution is able to penetrate the protective oxide and attack the metal layer at the bottom of the aperture and may even significantly erode the electrode metal layer 52 as indicated by the pitting 68a and 68b at the bottom of the aperture 56. This corrosive attack of the electrode layer sufficiently weakens the integrity of the hinge post to cause structural and/or electrical failures. Therefore, it would be advantageous to develop a fabrication process that does not result in damage to the structural integrity of the hinge post.